Method and apparatus for reducing excess sludge

ABSTRACT

The invention offers a method and apparatus for solubilizing excess sludge, having a high solubilizing ability, a low total cost, and capable of reducing the size of facilities. The method for reducing excess sludge comprises adding a solubilizing agent to excess sludge generated by microbial treatment of organic sewage, applying ultrasonic waves, applying a reduced pressure swelling step, then returning the result to the microbial treatment, whereby the volume of the excess sludge can be reduced. The apparatus for reducing excess sludge comprises means for adding a solubilizing agent to excess sludge generated by applying a microbial treatment to organic sewage, ultrasonic wave applying means for applying ultrasonic waves, and reduced pressure swelling means provided downstream of the ultrasonic wave applying means for applying a reduced pressure swelling process to the excess sludge.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for reducing the excesssludge generated in the biological treatment of various types of organicwaste fluid, and an apparatus for reducing excess sludge using thismethod.

BACKGROUND OF THE INVENTION

[0002] The organic waste fluid which is discarded in the form of varioustypes of industrial waste fluids as well as domestic sewage is primarilytreated by biological methods such as by using activated sludge. Sincethe microbes used can proliferate in abundance during this process,creating massive amounts of excess sludge whose disposal can present aproblem. As methods for handling this type of excess sludge, it has beenused for improving soil or as compost, but no fundamental solution hasyet been reached, so that in most cases, the excess sludge is dehydratedand incinerated or buried as industrial waste.

[0003] However, in recent years, such incineration has come to be seenas possibly posing a threat by generating toxic substances includingenvironmental hormones such as dioxins, and burial ultimately also givesrise to similar problems in the form of toxic substances which can leakout over time. Therefore, there has been a call for development oftechniques for reducing the excess sludge itself to provide a morefundamental solution to the problem, and many proposals have beenoffered.

[0004] Among these, the methods of Japanese Patent No. 2806495, JapanesePatent Application, First Publication No. H11-128975 and Japanese PatentApplication No. H11-218022 are relatively inexpensive and easy tocontrol. All of these methods involve the addition of alkalis to theexcess sludge followed by exposure to ultrasonic waves.

[0005] However, the method of Japanese Patent Application, FirstPublication No. H11-128975 is performed at a pH of 10.5 or less inconsideration of the cost of neutralization that is required whenreturning the solubilized excess sludge to the biological treatment, sothat the expected solubilization effect is difficult to achieve withthis method.

[0006] The method of Japanese Patent Application No. H11-218022 enablesthe pH to be raised to 12-13 by improving on the neutralizationprocedure, thus improving the solubilization effect, but a lot of timeis still required for the ultrasonic treatment, which requires highoutput and is therefore very expensive. Additionally, the ability ofmicrobes to be used for decomposition leaves something to be desired, tothe point of being difficult to put into actual practice. In connectionwith the art described above, there has been a strong demand fortechnological improvements that would allow for cost reductions andimproved effectiveness by obtaining the desired level of solubilizationeffects, and particularly the ability of microbes to be used fordecomposition, even with a low ultrasonic output and a short time ofapplication thereof.

SUMMARY OF THE INVENTION

[0007] The present invention has the object of offering a method andapparatus for solubilizing excess sludge, with a higher capacity forsolubilization than conventional methods, having a low overall cost andallowing equipment to be made smaller.

[0008] A method for reducing excess sludge according to claim 1 of thepresent invention is characterized by adding a solubilizing agent toexcess sludge generated by a microbial treatment of organic sewage;applying ultrasonic waves; applying a reduced pressure swellingtreatment; and returning the result to the microbial treatment, therebyreducing the volume of said excess sludge.

[0009] Due to this method, a high level of solubilization can beobtained by the reduced pressure swelling process, thus resulting in aparticularly high level of reusability by microbes, and achieving a highrate of reduction.

[0010] Another method for reducing excess sludge according to thepresent invention is characterized in that said solubilizing agent is analkali, a bacteriolytic agent, or a combination thereof.

[0011] According to this method, the alkalis contribute to thedissolution of proteins, are not harmful to the environment ifneutralized, and further in biological treatment systems, and can beused to adjust the pH prevent reductions in pH due to dissolution ofcarbon dioxide gas. While possible alkalis include, for example, NaOH,KOH, Mg(OH)₂ and Ca(OH)₂, other compounds may be used as long as theyare capable of raising the pH.

[0012] Additionally, the bacteriolytic agent has the function ofdestroying the cell walls of microbes. While there are many types ofbacteriolytic agents, the type does not matter as long as it is capableof decomposing bacteria. Examples include hydrogen peroxide, dichloroussoda and ozone.

[0013] An apparatus for reducing excess sludge according to anotherembodiment of the present invention is characterized by comprising meansfor adding a solubilizing agent to excess sludge generated by microbialtreatment of organic sewage; ultrasonic wave applying means for applyingultrasonic waves; and reduced pressure swelling means, provideddownstream of the ultrasonic wave applying means, for applying a reducedpressure swelling process to the excess sludge.

[0014] According to this embodiment, a high rate of solubilization isachieved by the reduced pressure swelling means, thus resulting in aparticularly high level of reusability by microbes, and achieving a highrate of reduction.

[0015] An apparatus for reducing excess sludge according to anotherembodiment of the present invention is characterized in that the reducedpressure swelling means is a homogenizer.

[0016] According to this embodiment, solubilizing agents such as alkalisand ultrasound are used to destroy or damage the cell walls of microbes,and a reduced pressure swelling process causes the contents of the cellsto leak out, after which further reactions by solubilizing agents causesthem to be converted to substances which can be handled by microbes.Whereas the solubilization effect due to the reduced pressure swellingcan be raised by raising the pressure of the homogenizer, the optimumoperating conditions may be determined in consideration of themanufacturing cost and running cost of the apparatus.

[0017] An apparatus for reducing excess sludge according to anotherembodiment of the present invention is characterized in that saidultrasonic wave applying means and said reduced pressure swelling meansare constructed as separate parts, the ultrasonic wave applying meansand reduced pressure swelling means being serially connected directly orthrough the medium of other equipment.

[0018] According to this embodiment, the ultrasonic wave applying meansand the reduced pressure swelling means are separate parts, thusallowing for a design which maximizes the performance of the respectiveparts. Furthermore, the operating conditions of the processes ofapplication of ultrasonic waves and reduced pressure swelling can befreely controlled independently for respective optimization.

[0019] An apparatus for reducing excess sludge according to yet anotherembodiment of the present invention is constructed as a single apparatushaving the functions of both said ultrasonic wave applying means andsaid reduced pressure swelling means.

[0020] According to this embodiment, the ultrasonic wave applying meansand reduced pressure swelling means are constructed as a singleapparatus, thus allowing for a design capable of holding the size of theapparatus to a minimum. Additionally, the cost of the apparatus overallcan be reduced, thus enabling the apparatus to be easily controlled aswell.

[0021] An apparatus for reducing excess sludge according to yet anotherembodiment of the present invention is characterized in that theultrasonic wave applying means includes an ultrasonic vibrator, and saidreduced pressure swelling means comprises a plate having a plurality ofthrough holes formed therein, provided at a position downstream of saidultrasonic vibrator such as to intersect flow.

[0022] According to this embodiment, the plate (porous plate) positionedagainst the flow gives rise to the reduced pressure swelling effect,thereby ensuring a high level of solubilization.

[0023] An apparatus for reducing excess sludge according to yet anotherembodiment of the present invention is characterized in that aflow-receiving plate is provided between said ultrasonic wave applyingmeans and said reduced pressure swelling means so as to obstruct flow.

[0024] According to this embodiment, the flow-receiving plate that isprovided on the flow path increases the ultrasonic wave cavitationeffect, changes the flow of the solution, and aids in mixture, so that ahigh rate of solubilization can be obtained even if the plate (porousplate) is given a larger hole diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is diagram showing a method for reducing excess sludgeaccording to an embodiment of the present invention.

[0026]FIG. 2 is a schematic diagram showing an excess sludge reducingapparatus according to an embodiment of the present invention.

[0027]FIG. 3 is a schematic diagram showing an excess sludge reducingapparatus according to an embodiment of the present invention.

[0028]FIG. 4 is a schematic diagram showing an excess sludge reducingapparatus according to an embodiment of the present invention.

[0029]FIG. 5 is a schematic diagram showing an excess sludge reducingapparatus according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] Herebelow, embodiments of the present invention shall bedescribed with reference to the drawings.

[0031]FIG. 1 shows a flow chart of an example of a sewage treatmentmethod including the excess sludge reducing method according to anembodiment of the present invention. In FIG. 1, raw fluid 1 which isbasically organic sewage is passed through an initial precipitation pool2 and a flow regulating pool 3, then flows through a biologicaltreatment tank 4 where it is treated by microbes, after which it isseparated into solid and liquid parts in a final precipitation pool 5 toform treated water 6 and sludge (return sludge 7 and excess sludge 8). Aportion of this sludge is returned to the biological treatment tank asreturn sludge 7, while the rest is taken as excess sludge, which may ormay not be passed through a sludge concentrating tank 10, and isdirected to a sludge solubilizing process DV 11 where the excess sludge8 is solubilized and broken down. The solubilized sludge 9 is returnedto the flow regulating tank 3 or biological treatment tank 4 or both inthe sewage treatment process, and the solubilized sludge 9 is thenbroken down by microbes in the biological treatment tank 4.

[0032] The excess sludge 8 which is introduced into the sludgesolubilizing process DV 11 is processed with an ultrasonic vibrator, andalso undergoes a reduced pressure swelling process by a homogenizer forthe solubilization treatment.

[0033] The sludge solubilization process DV 11 has the functions of bothultrasonic wave applying means and reduced pressure swelling means inthe form of the homogenizer. This structure is shown in FIGS. 2-5.

[0034]FIGS. 2 and 3 show embodiments in which the ultrasonic waveapplying means and the reduced pressure swelling means (homogenizer) areseparated and connected serially. In the drawing, 12 denotes anultrasonic vibrator as a part of the ultrasonic wave applying means, 13denotes an ultrasonic treatment tank, 14 denotes a pump, 15 denotes areduced pressure swelling treatment tank, and 16 denotes a homogenizerused as reduced pressure swelling means.

[0035]FIGS. 4 and 5 show a single apparatus having the functions of bothultrasonic wave applying means and reduced pressure swelling means(homogenizer). Those wherein these means are manufactured as separateparts, then assembled together by means of fastening means such as boltsand nuts shall be considered to be included among the single apparatusmentioned above.

[0036]FIG. 4 shows an embodiment wherein a porous plate 26 is placedimmediately before the ultrasonic vibrator 22 in the direction ofapplication of the ultrasonic waves, so that the excess sludge which hasbeen exposed to the ultrasonic waves is sent to the porous plate 26 dueto the propagation of the ultrasonic waves and the pressure from thepump.

[0037] Due to the employment of this structure, the pressure on theprimary side of the porous plate 26 is high, but the pressure on thesecondary side of the porous plate 26 is close to atmospheric pressure,so that the liquid which has passed through the porous plate 26immediately lowers to a pressure close to atmospheric pressure. Thispressure difference causes the microbes in the excess sludge to swelldue to the sudden pressure drop, so that the bacteria which haveundergone cavitation under the microscopic high frequency waves of theultrasonic vibrations then undergo a swelling effect of the liquiditself, the destruction of the cell walls causing leakage of the contentof the cells under further swelling effects, and finally being treatedby solvents such as alkalis to form substances that are easily brokendown by microbes. Additionally, the cell walls which have been damagedby the action of the alkali solvent and ultrasonic waves are completelydestroyed by the reduced pressure swelling, thus promoting the leakageof the fluids contained therein.

[0038]FIG. 5 shows an embodiment in which a flow-receiving plate 27 isplaced immediately before the ultrasonic vibrator 22.

[0039] By employing this structure, the solution which has been treatedby the ultrasonic waves is kept at a high pressure by the flow receivingplate 27, passes through the passages to the sides of the flow-receivingplate 27, is relieved of the pressure at the exits to the cell (porousplate 26), and there undergoes swelling so as to destroy the cell wallsand promote leakage of their content.

[0040] The exit of the cell may be a porous plate, a showerhead typeplate or a single circular hole. Of these, the showerhead type plate andsingle circular hole are not easily clogged, so that even if insolublematter other than microbes are mixed therein, there is no need for aforeign matter removing step preceding this treatment, and the operationcan be performed stably. When considering overall factors such as clogsand the reduced pressure swelling effect, it is desirable to use ashowerhead plate.

[0041] Additionally, the above-mentioned flow-receiving plate 27 may beflat or of arcuate shape. While it is desirable to provide aflow-receiving plate 27, under special circumstances, a certain degreeof effectiveness can be achieved without providing a flow-receivingplate 27.

[0042] In this way, with the method and apparatus for reducing excesssludge according to the present invention, the solubilization effect israised by applying ultrasonic waves under alkaline conditions, thenapplying a homogenizer, or applying these processes simultaneously. Thatis, the synergistic effects of the ultrasonic waves and homogenizer areable to raise the solubilization effect of the microbial cells. Here,the homogenizer is an apparatus which passes the subject materialthrough a porous plate under pressurized conditions and instantlydepressurizes the material, thereby destroying the cells of microbes andcausing the content of the cells to leak out.

[0043] The process of destroying or damaging the cell walls of microbeswith the application of alkalis and ultrasonic waves, causing thecontent of the cells to leak outside the cells and converting them tosubstances which can be handled by microbes is important. In order toperform this step, the pressure of a partially solubilized liquid israised and instantly depressurized, causing the cells to swell, so thatthe contents of cells with damaged or destroyed cell walls will spillout of the cells, to be acted on by the alkalis to change intosubstances that are readily decomposed by microbes.

[0044] In some cases, the viscosity of the excess sludge can be raisedby the action of the alkalis and ultrasonic waves, thereby increasingthe resistance to microscopic movements of substances in the liquid,thereby lowering the susceptibility of the partially solubilized cellsand liquid itself (alkali liquid) to mixture, making solubilization byfurther decomposition difficult so that the solubilization effectscannot be improved. In order to overcome this problem, the homogenizeris used to obtain a synergistic effect. While alkalis have been usedabove, similar effects can be obtained using other types ofbacteriolytic agents.

[0045] Additionally, the solubilized sludge can then be returned to theformer stages of the biological treatment process, to be broken down byother microbes to reduce the excess sludge.

[0046] Herebelow, the method and apparatus for reducing excess sludgeaccording to the present invention shall be described in detail withreference to examples.

EXAMPLE 1

[0047] Solubilization tests were performed on sludge (from a foodprocessing factory) obtained during treatment of organic sewage withaerobic microbes under the following conditions. Sludge Concentration;10050 mg/liter, pH 6.3 pH Adjuster: NaOH Initial Solubilization pH: 12Ultrasonic Frequency: 19 Hz, Output 400 W, Retention Time 1 min (usingequipment produced by Seidensha Electronics Co., Ltd.)

[0048] As an indicator of solubilization, the sludge was separated in acentrifuge (10 min at 4000 rpm), and judged on the basis of measurementsof the increase in COD concentration in the sludge using the recoveredfluid in measurements of the oxygen consumption rate by potassiumdichromate (COD_(cr)) at 150° C. (in accordance with the JIS K0102standard). This measurement was made using a colorimeter and COD reactormanufactured by HACH Corporation. TABLE 1 A B C D E F COD 255 1880 52056560 6860 6425 (mg/l)

[0049] A: Only homogenizer

[0050] B: Alkali homogenizer.

[0051] C: Alkali ultrasound.

[0052] D: Alkali ultrasound+serially connected homogenizer (FIG. 3).

[0053] E: Alkali ultrasound+integral porous-plate homogenizer (FIG. 4).

[0054] F: Alkali ultrasound+integral showerhead homogenizer (FIG. 5).

[0055] Table 1 shows the results of solubilization using the excesssludge from a food processing factory. The homogenizers of thestructures shown in FIGS. 3, 4 and 5 were respectively used. The porousplates of FIGS. 3 and 4 had pores with a diameters of 1.5 mm, with aporosity of 38%, with the showerhead plate of FIG. 5 having six holeswith a diameter of 4.5 mm each. For the purposes of comparison, theresults for the cases where no homogenizer is used, where only thehomogenizer is used and only alkalis are used are also shown. Thepressure of the pump sending liquid to the homogenizers was 4.0-6.0kgf/cm².

[0056] As can be seen in Table 1, the solubilization effects improveddramatically when using a homogenizer as opposed to cases in which ahomogenizer was not used. Additionally, it can be seen that thesolubilization effects are not as significant when using only ahomogenizer. While the showerhead type plate gave poorer results thanthe porous plate, it still demonstrated a marked improvement over thecase where only ultrasonic waves are used. Additionally, the fact thatthe integral type apparatus gave better results than the separate serialtype is notable. This is believed to be due to the presence ofresistance against direct flow. In any case, Table 1 serves todemonstrate that the COD changes in sludge which has undergone a reducedpressure swelling effect in a homogenizer.

EXAMPLE 2

[0057] A biological treatment experiment was performed to turn thesewage from the same food processing factory into water in accordancewith the flow chart of FIG. 1. The experiment was performed by sendingfluid which had been solubilized by ultrasonic waves and a homogenizerat room temperature and alkaline conditions (initially ph 12) to a fluidrate regulating tank without adjusting the pH, and observing the effectsof the biological treatment. The results are shown in Table 2.

[0058] The testing conditions were as follows: Water Flow Rate: 50liters/day Water BOD: 1050 mg/liter Water Flow Regulating Tank: 50 literAmount of Sludge to Solubilization: 3.2 times amount of excess sludgeSolubilizing Agent: NaOH Ultrasound Exposure Time: 1 minute

[0059] TABLE 2 O A B C D E F Quality of Treated Water BOD (mg/l) 7.5 7.57.2 7.3 7.3 7.6 7.4 COD (mg/l) 7.5 8 10 12 14 14.2 14.1 SS (mg/l) 11 1111.5 11.4 11.3 11.2 11.3 pH 7.2 7.2 7.3 7.3 7.3 7.3 7.3 BOD Sludge 0.440.42 0.31 0.14 0.04 0.02 0.02 Conversion Rate (g-SS/g-BOD)

[0060] O represents an experiment made using a test apparatus withoutthe solubilization step in the flow chart of FIG. 1, while the othersare the same as their counterparts in Table 1.

[0061] As shown in Table 2, the results for the case of solubilizationwith only a homogenizer in the pressure range of 4.0-6.0 kgf/cm² arealmost the same as those for the case where there is no solubilizationstep, and this is clearly due to the fact that there is no sludgereducing effect. While significant solubilization effects did appear inthe case of the alkali homogenizer, the expected level of effects werenot obtained. While there was a considerable reducing effect with alkaliultrasound, the BOD sludge conversion rate was still only 0.14. On theother hand, when applying ultrasonic waves under alkali conditions, thenapplying the homogenizer, in all cases the sludge generation rate wasmuch lower than in the case of alkali ultrasound, and there was almostno difference in the treated water quality.

[0062] Upon comparison of the results of the respective experimentsshown in Table 2 with Table 1, when taking the COD as the solubilizationindicator for evaluating the solubilization, there is a solubilizationeffect in the case of a homogenizer with alkali ultrasound, but whentaking the rate of conversion of the sludge as the indicator forevaluating the sludge reduction rate, the effect of the homogenizer isshown to be considerably large. This is believed to be due to the factthat the homogenizer promotes leakage of the content from the cell wallsof microbes, thus assisting in biological treatments by furtherdecomposition. From these results, the synergistic effect of alkaliultrasound and the homogenizer was confirmed to have a major impact onthe sludge reduction effect.

[0063] As explained above, according to the method for reducing excesssludge according to the present invention, the solubilization effect canbe raised by applying ultrasound under alkaline conditions, thenapplying a reduced pressure swelling process in a homogenizer, orapplying these processes simultaneously. That is, by synergizing theaction of the ultrasound with the action of the homogenizer, the effectof solubilization of microbial cells can be increased.

[0064] According to a method for reducing excess sludge based on anotherembodiment of the present invention, the alkalis contribute to thedissolution of proteins, are not harmful to the environment ifneutralized, and further in biological treatment systems, and can beused to adjust the pH prevent reductions in pH due to dissolution ofcarbon dioxide gas.

[0065] According to an apparatus for reducing excess sludge based onanother embodiment of the present invention, a high solubilization rateis ensured by reduced pressure swelling means, and the resulting highsolubilization effect enables a high rate of reduction to be attained.

[0066] According to an apparatus for reducing excess sludge based onanother embodiment of the present invention, solubilizing agents such asalkalis and ultrasound are used to destroy or damage the cell walls ofmicrobes, and a reduced pressure swelling process causes the contents ofthe cells to leak out, after which further reactions by solubilizingagents causes them to be converted to substances which can be handled bymicrobes.

[0067] According to an excess sludge reducing apparatus based on anotherembodiment of the present invention, ultrasonic wave applying means andreduced pressure swelling means are separate apparatus, so that they canbe designed for maximum performance of the respective apparatus, and theoperating conditions of the ultrasonic wave application and reducedpressure swelling processes can be independently controlled for optimumresults.

[0068] According to an excess sludge reducing apparatus based on anotherembodiment of the present invention, ultrasonic wave applying means andreduced pressure swelling means are provided in a single apparatus, sothat the size of the apparatus can be designed to be made as compact aspossible, thereby reducing the cost of the apparatus overall, andenabling the apparatus to be easily controlled.

[0069] According to an excess sludge reducing apparatus based on anotherembodiment of the present invention, a high solubilization rate isobtained by a plate placed to obstruct the flow.

[0070] According to an excess sludge reducing apparatus based on yetanother embodiment of the present invention, a high solubilization rateis obtained by a flow receiving plate which is provided on the flowpath.

1. A method of reducing excess sludge, characterized by adding asolubilizing agent to excess sludge generated by a microbial treatmentof organic sewage; applying ultrasonic waves; applying a reducedpressure swelling treatment; and returning the result to the microbialtreatment, thereby reducing the volume of said excess sludge.
 2. Anexcess sludge reducing method in accordance with claim 1, wherein saidsolubilizing agent is an alkali, a bacteriolytic agent, or a combinationthereof.
 3. An apparatus for reducing excess sludge, characterized bycomprising means for adding a solubilizing agent to excess sludgegenerated by microbial treatment of organic sewage; ultrasonic waveapplying means for applying ultrasonic waves; and reduced pressureswelling means, provided downstream of the ultrasonic wave applyingmeans, for applying a reduced pressure swelling process to the excesssludge.
 4. An apparatus for reducing excess sludge in accordance withclaim 3, wherein said reduced pressure swelling means is a homogenizer.5. An apparatus for reducing excess sludge in accordance with claim 3 or4, wherein said ultrasonic wave applying means and said reduced pressureswelling means are constructed as separate parts, the ultrasonic waveapplying means and reduced pressure swelling means being seriallyconnected directly or through the medium of other equipment.
 6. Anapparatus for reducing excess sludge in accordance with claim 3 or 4,constructed as a single apparatus having the functions of both saidultrasonic wave applying means and said reduced pressure swelling means.7. An apparatus for reducing excess sludge in accordance with claim 6,wherein said ultrasonic wave applying means includes an ultrasonicvibrator, and said reduced pressure swelling means comprises a platehaving a plurality of through holes formed therein, provided at aposition downstream of said ultrasonic vibrator such as to intersectflow.
 8. An apparatus for reducing excess sludge in accordance withclaim 6 or 7, wherein a flow-receiving plate is provided between saidultrasonic wave applying means and said reduced pressure swelling meansso as to obstruct flow.